The present invention relates to semiconductor integrated circuit devices with variable gain amplifiers, and, more particularly, to semiconductor integrated circuit devices that have auto gain controllers and orthogonal modulators.
To decrease power consumption and improve communication quality in a mobile communication device such as a cellular phone, a transmitting portion of the communication device must be provided with an improved orthogonal modulator.
FIG. 1 shows a prior art semiconductor integrated circuit device 50 that includes an auto gain controller (AGC) 6 and an orthogonal modulator 1. The orthogonal modulator 1 has a phase shifter circuit 2, first and second modulation mixer circuits 3, 4, and an adder 5.
The phase shifter circuit 2 receives complementary input signals LOin that have a predetermined frequency. The phase shifter circuit 2 shifts the phase of each input signal LOin at positive or negative 90 degrees to generate first and second carrier signals. The phase of the first carrier signal is thus offset from the phase of the second carrier signal at 180 degrees.
The first modulation mixer circuit 3 receives the first carrier signal and base-band signals Q, XQ. The second modulation mixer circuit 4 receives the second carrier signal and base-band signals I, XI.
The first modulation mixer circuit 3 multiplies the first carrier signal by the base-band signals Q, XQ to generate a modulation signal. The modulation signal is supplied to the adder 5. The second modulation mixer circuit 4 multiplies the second carrier signal by the base band signals I, XI to generate a modulation signal. The modulation signal is also supplied to the adder 5.
The adder 5 adds the modulation signals of the first and second modulation mixer circuits 3, 4 to generate a sum signal RFout. The adder 5 then sends the sum signal RFout to the AGC 6.
The AGC 6 includes an auto gain control circuit (AGC circuit) 7 and a gain adjusting circuit (CNT circuit) 8. The sum signal RFout is sent to the AGC circuit 7.
The CNT circuit 8 generates an AGC gain control signal Vagc in accordance with a main control signal Vcnt and sends the AGC gain control signal Vagc to the AGC circuit 7.
As shown in FIG. 3, the gain G1 of the phase shifter circuit 2 remains constant regardless of the AGC gain control signal Vagc (the main control signal Vcnt). In contrast, the gain G2 of the AGC circuit 7 varies in relation to the AGC gain control signal Vagc. Thus, the total gain G3 of the orthogonal modulator 1 and the AGC 6 varies in relation to the main control signal Vcnt. Accordingly, in the graph of FIG. 3, the line that represents the total gain G3 and the line that represents the gain G2 of the AGC 6 are inclined at equal gradients.
The AGC circuit 7 generates an output signal OUT in correspondence with the level of the main control signal Vcnt. As shown in FIG. 2, if the level of the main control signal Vcnt varies from Vcnt1 to Vcnt2, the output level Pout of the output signal OUT decreases from a maximum value Pmax to a minimum value Pmin. In other words, the output level Pout is adjusted in relation to the level of the main control signal Vcnt.
In the semiconductor integrated circuit device 50, the output signal OUT constantly includes an output frequency component of the phase shifter circuit 2 as a carrier leak CL. For example, as shown in FIG. 2, if the gain of the AGC circuit 7 decreases, the carrier leak CL is attenuated together with the output level Pout. However, as the output level Pout decreases toward the minimum level Pmin, the decrease rate of the carrier leak CL becomes smaller than that of the output level Pout. In other words, as the level of the main control signal Vcnt decreases toward the level Vcnt2, the interval between the curve that represents the carrier leak CL and the curve that represents the output level Pout becomes smaller.
As shown in FIG. 2, when the output level Pout is the maximum level Pmax, the level difference between the output signal OUT and the carrier leak CL is xcex94CLa. When the output level Pout is the minimum level Pmin, the level difference between the output signal OUT and the carrier leak CL is xcex94CLb. The level difference xcex94CLb is smaller than the level difference xcex94CLa. In other words, the level of the output frequency component (the carrier leak component) of the phase shifter circuit 2 becomes constant before the output level Pout reaches the minimum level Pmin.
Thus, the carrier leak characteristics of the semiconductor integrated circuit device 50 are changed if the output level Pout is lowered toward the minimum level Pmin in accordance with the main control signal Vcnt.
Accordingly, it is an objective of the present invention to provide an orthogonal modulator that maintains carrier leak characteristics regardless of attenuation of an output signal level.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, the invention provides a semiconductor integrated circuit device including an orthogonal modulator for generating a modulation signal. The orthogonal modulator includes a phase shifter circuit. An auto gain controller is connected to the orthogonal modulator for amplifying the modulation signal to generate an amplified modulation signal. A gain adjusting circuit adjusts a gain of the phase shifter circuit in accordance with a control signal.
In an embodiment of the present invention there is provided a semiconductor integrated circuit device including a phase shifter circuit for receiving a plurality of complementary input signals that have a predetermined frequency and shifting a phase of each complementary input signal to generate first and second carrier signals. A first modulation mixer circuit is connected to the phase shifter circuit to multiply the first carrier signal by a first base-band signal and generate a first modulation signal. A second modulation mixer circuit is connected to the phase shifter circuit to multiply the second carrier signal by a second base-band signal and generate a second modulation signal. An adder is connected to the first and second modulation mixer circuits to add the first and second modulation signals and generate a sum signal. A first gain adjusting circuit is connected to the phase shifter circuit to control the amplitude of each carrier signal of the phase shifter circuit in accordance with a control signal. An auto gain controller is connected to the adder to generate an amplified modulation signal from the sum signal. A second gain adjusting circuit is connected to the auto gain controller to control a gain of the auto gain controller in accordance with the control signal.
In an embodiment of the present invention there is provided a semiconductor integrated circuit device including an analog/digital converter for converting an analog control signal to a digital control signal. First and second phase shifter circuits are connected to the analog/digital converter to receive the digital control signal and a complementary input signal that has a predetermined frequency. Either the first or second phase shifter circuit is activated in response to the digital control signal. The first phase shifter circuit generates a carrier signal in accordance with a relatively small gain when activated. The second phase shifter circuit generates the carrier signal in accordance with a relatively large gain when activated. A quadrature modulator is connected to the first and second phase shifter circuits to generate a modulation signal from the carrier signal. An auto gain controller is connected to the quadrature modulator to amplify the modulation signal in accordance with a predetermined gain and generate an amplified modulation signal. First and second gain adjusting circuits are connected to the auto gain controller and the analog/digital converter to receive the analog control signal and the digital control signal and adjust the gain of the auto gain controller. The first or second gain adjusting circuit is selectively activated in response to the digital control signal. The first gain adjusting circuit is activated together with the first phase shifter circuit to operate the auto gain controller in accordance with a relatively high gain. The second gain adjusting circuit is activated together with the second phase shifter circuit to operate the auto gain controller in accordance with a relatively low gain.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.